The present invention relates in general to methods for producing ceramic insulation by introducing very small-sized porosity in the fired ceramic in a controlled fashion, and production of multilayered, multifunctional ceramic components. Further, the present invention includes methods to produce net-shape and/or net-size ceramic insulation components. The present invention would be applicable for high temperature applications as insulation or refractory components. The insulation can be designed for both load-bearing (structural) and non-load bearing applications.
The availability of a high alumina, silica-free, fiber-free insulation with fine porosity will fulfill unmet needs for various industries.
In the solid oxide fuel cell industry, silica in the insulation surrounding the fuel cell is detrimental for the long-term performance of the solid oxide fuel cells (SOFC). Silica, in the presence of humidity and high temperatures converts into a gaseous form SiO, which may then react and adversely affect the fuel cell performance. Therefore, a silica-free, oxide-based insulation is desired for SOFC. Currently, there are only two alternative commercially-available products that meet the need for high alumina, silica-free insulation for solid oxide fuel cells. Both have significantly high cost. Thus, it would be an advancement in the art to provide a lower cost high alumina insulation to fuel cell manufacturers.
In the furnace industry, fiber-based (non-load bearing) insulation is the most commonly used high temperature insulation. Alumina-fiber insulation is used in reducing environments or where there is high humidity and/or high temperature requirements. However, manufacture and use of high alumina fiber-based insulation is often associated with health concerns and high product cost. Thus, it would be an improvement in the art if an alumina-based fiber-free insulation is available with the additional benefit of lower cost.
Most of the oxide based, silica-free, structural (load-bearing) insulation have a density of 2.5-3.0 g/cc and a flexural strength of less than <10 MPa (1450 psi). Further, the flexural strength of the insulation quickly degrades (sometimes by as much as 50%) when exposed to temperatures above 600° C. i.e. they have poor hot strength. It would be an improvement in the art if a new silica-free insulation is available that has a higher strength-to-weight ratio over conventional insulation products currently available in the market and which also has good mechanical and thermo-chemical stability at the operating temperatures. High strength-to-weight ratio ceramic components can be used in a wide range of applications due to their insulative properties. These applications can be in diesel particulate filters (DPF) as ceramic honeycomb structures, or diesel particulate filter frame materials, or DPF mount supports where lightweight and good strength are critical. Other applications can be ceramic hot gas filters, and supports or carriers for catalyst where tailored micro-porosity and strength is critical to their lifetime performance. It would be an improvement in the art to provide new alternative compositions that are lightweight, possess good hot strength, and which also provide flexibility in designing appropriate pore structures.
Traditional ceramic processing calls for sintering and machining of components after firing. For example, in solid oxide fuel cell insulation, significant machining is performed to obtain a tight insulation fit around the fuel cell stack. As a result, machining cost could be over 50% of the overall insulation cost. Similarly, high tolerance ceramic diesel particulate filters are prepared via extrusion followed by a high temperature sintering step which results in 10-20% shrinkage. In applications where high dimensional tolerances are required after firing, shrinkages of 10-120% need to be controlled very precisely, thus increasing the complexity and risk in the manufacturing processes. In contrast, net-shape and/or net-size processes could virtually eliminate these issues and minimize the concerns associated with firing shrinkage and post-machining steps. It would be an improvement in the art to develop a ceramic that can be net-shape and/or net-size. Such an improvement would significantly reduce the post-machining requirements and the associated costs.
Most commonly available high alumina insulation and refractory components have large sized pores (20 micron-500 micron). For a given total pore volume, larger pore sizes result in a less effective thermal insulator and decrease the component's overall strength. In fact, some of the best silica-based insulations have pore sizes in the nanometer range. However, due to the presence of silica, their maximum temperature use is often below 1000° C. and they cannot be used in reducing or humid environments. Furthermore, it is desired in the metal melting industry that the ceramic components and consumables have a fine pore size (<10 micron) since it improves the ceramic component's lifetime performance, i.e., a smaller pore limits the molten melt penetration into the ceramic liner surrounding the melt. But since the starting particle size of most the raw materials used to make refractory ceramic components is between 20 microns-500 microns, the corresponding pore sizes range from 10 micron-250 micron. Thus there is a need for high alumina insulation with superior thermal and lifetime performance, and it would be an improvement in the art to design very fine pores into the alumina matrix, analogous to the pore sizes observed in the silica-based insulation.
Conventional ceramic processing usually involves monolithic components where the bulk of the component just serves one functional purpose. However, multifunctional designs are slowly being realized as a growing trend in ceramics. Such multifunctional designs may include two or more layers where each layer exists for a specific purpose. For example, in the aluminum melting industry, the topmost layer in contact with the aluminum melt can be silicon carbide for good chemical inertness while the bottom layer can be alumina for good insulation. However, due to different thermal characteristics of individual ceramics, making such layered composite structures is often challenging. It would be an advancement in the art to provide a cost effective process to manufacture multi-layered ceramics components that have multifunctional properties.
The conventional method of making alumina-based phosphate-bonded products involves reacting a source of aluminum and phosphate at elevated temperatures, often above 100° C. The process requires heating the reactants to a high enough temperature (>100° C.) to initiate and propagate the reaction. As a result, the process requires expensive and specialized equipment making it somewhat difficult and inconvenient to manufacture phosphate-bonded products. One reason why high temperatures are needed is that the aluminum source, which is primarily alumina-based or aluminum hydroxide-based, consist of large grains (>10 micron to ≦500 micron) which are inert to the phosphate reagents at room temperature. Often, the reactant mixture simply consists of a mixture of alumina-based (or aluminum hydroxide-based) powder, a few other relevant inorganic ingredients as secondary phases, water and phosphate-based reagents (with minor amounts of shelf life preserving agents).
Such conventional approaches for manufacturing phosphate-bonded products predominantly exclude the use of organics in the form of organic dispersants, binders, or plasticizers. For example, dispersing the alumina-based slurry in solvent (prior to the addition of the phosphate-based reagent) and prior to the subsequent shaping processes, is not practiced in conventional methods of processing phosphate-bonded ceramics. The larger grains used in conventional approaches offer a further disadvantage that they are nearly impossible to disperse and have a strong tendency to settle out of the suspension thereby quickly nullifying any positive effects that organic dispersing aids might provide. Further, most ceramic powders consist of aggregates and agglomerates. The problem of agglomeration is especially more pronounced at lower particle sizes (1 micron to 20 micron).
If the powders agglomerate into hard-to-break aggregates and are in an un-dispersed state, then only the particles on the surface of the aggregate will react with the reagent. Consequently, the large grains, agglomerates of finer particles, and a poorly dispersed or hard to disperse ceramic suspension necessitate the use of significantly higher concentrations of phosphate-based reagent coupled with higher temperatures (>100° C.) or high energy mixing in order to achieve complete reaction with all the ceramic particles.
In conventional approaches of manufacturing phosphate-bonded alumina products, the molar ratio of aluminum to phosphorus is often 1:1 or lower (i.e., the reaction environment is significantly phosphate rich). Consumption, handling and disposal or phosphoric acid is an environmentally sensitive matter. It would, therefore, be an improvement in the art if phosphate-bonded reaction products can be made at lower temperatures and with much less phosphate-based reagents without sacrificing the desired properties of the final product.